1. Field of the Invention
The present invention relates to a glass for a substrate having high resistance against progress of fracture i.e. a glass for a substrate having high fracture toughness.
2. Discussion of Background
In recent years, large sized flat display panels represented by color plasma display panels (hereinafter referred to as color PDP) have been increasingly used, and glasses to be used as substrates thereof have been diversified. Heretofore, usual soda lime silica glass has been widely used for substrates for large sized flat display panels. One of the reasons is such that the thermal expansion coefficients of various glass frit materials to be used as constituting part materials for panels, including inorganic sealing materials, can easily be adjusted to the thermal expansion coefficient of soda lime silica glass.
On the other hand, in order to reduce deformation or thermal shrinkage of glass substrates in the heat treatment process during the production of large sized flat display panels, it is strongly desired to improve the thermal resistance of the glass for substrates. For this purpose, a so-called high strain point glass has been widely used as a substrate, which has a thermal expansion coefficient of the same level as soda lime silica glass and has a higher strain point (at least about 550.degree. C.) and which has an alkali content controlled to be low in order to improve the electrical insulating property.
Further, a glass substrate is being practically employed as a substrate for information recording media, particularly as a substrate for magnetic discs (hard discs), since it is excellent in the surface smoothness or the mechanical strength such as impact resistance.
However, such a high strain point glass is brittle as compared with soda lime silica glass and thus has had a problem that it is likely to break in the production process. Further, such a high strain point glass has had another problem that its density is large, whereby it has been difficult to reduce the weight of the large sized flat display panel.
To solve such problems, a glass for a substrate has been proposed which has a low density and is hardly scratched, and which is suitable as a substrate for a flat panel display (JP-A-9-301733). The characteristic of being hardly scratched is effective in a case where a flaw as a fracture origin is likely to be imparted in the process for producing a panel, but it is not necessarily effective in a case where a flaw is imparted during processing treatment such as cutting prior to such a production process. It is usual that many flaws which are likely to be fracture origins, are already present at an edge portion of the glass in the processing treatment such as cutting.
Further, also in such a case where glass is employed as a substrate for magnetic discs, many processing treatments are required such as circular processing, center cut, inner and outer peripheral edge chamfering, etc. During such processing treatments, many flaws which are likely to be fracture origins, will be formed along the edge, etc., of the glass, and there has been a problem that not only during the production process, but also during the use of magnetic discs, the glass is likely to break from such flaws as fracture origins. Recently, it has been desired to increase the speed of rotation of discs in order to improve the read/write speed, and especially in such a case, the problem of fracture of glass becomes more serious.
In such a case, in order to prevent fracture of the glass, it has been necessary to present a glass for a substrate which essentially has high resistance against the progress of fracture due to a tensile stress, i.e. a glass for a substrate having high fracture toughness.